The Galilean Moons of Jupiter The Galilean Moons of Jupiter Summary: Jupiter has more than 60 known moons, but understanding the geology of its four largest will hopefully lead to some groundbreaking discoveries. Sections: The Galilean Moons Each of the Jovian planets has a number of moons, although Jupiter has the most with more than 60 catalogued to date. Jupiter's 4 largest moons exhibit some of the most interesting geology in the solar system. They were discovered by Galileo Galilei and are known as the Galilean moons. Jupiter's largest moon, Ganymede, is larger than Mercury while the other three are larger than Pluto. Outward from Jupiter: Io: Volcanoes and All Jupiter's Moon Io The Voyager spacecraft took the first close-up images of Io more than 300 years after the moon's discovery. The images showed a surface with no signs of craters from past impacts. What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress.

The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system. Io Close-Up (click to enlarge) Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red. Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes. Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics. The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Europa: What Lies Beneath? Europa Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old.

Europa's Interior Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle. Surrounding the rocky interior appears to be an icy layer 100 kilometers thick, the top few kilometers of which seem to be frozen solid. The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Analyzing Europa's Cracked Surface Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice. Other evidence comes from double-ridged cracks on the surface. Tidal flexing that allows water to well up and build ridges may create these cracks. Ganymede: Largest Moon in the Solar System Two Regions of Ganymede's Surface (click to enlarge) The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age.

The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions. If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa? Callisto: The Outermost Galilean Moon Callisto Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior. Callisto's Surface Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans. Europa Moon - Europa Moon Of Jupiter The Facts The orbit is shaped like an oval, not a circle.

At its closest point Europa is 412,835 miles (664,792 kilometres) from the planet and at its farthest point it is 420,670 miles (677,408 kilometres) from the planet. When Europa moves closer to Jupiter, Jupiter’s gravity causes a higher tide in the sea beneath the ice, which probably causes the ice to crack under the surface. Figure 3. The icy surface of Europa showing the effects of cracks under the surface ice. This movement in the salt water causes it to be less cold than you would expect – Europa is five times as far from the Sun as the Earth. Some scientists believe that some form of life could exist in the salt oceans of Europa. Like the other Galilean moons, Europa was named after a figure from Greek mythology. Europa was the daughter of the King of Tyre and, like Io, was loved by the Greek sky god, Zeus, who was known to the Romans as Jupiter. Zeus turned himself into a beautiful and gentle white bull that came out of the sea onto the beach where Europa was playing with her friends.

Ganymede (Moon) Facts Ganymede Eris Ganymede Moon Facts Ganymede is Jupiter’s largest moon and also the largest moon in the solar system. Ganymede was discovered in 1610 by astronomer Galileo Galilei and is named for the mythical Greek son of a King who was carried to the sky by Zeus posing as an eagle. Ganymede Moon Profile 1.48 x 10^23 kg (2.0 Moons) Orbits: Ganymede size compared to the Moon and Earth Jupiter’s Galilean moons size comparison (Io, Europa, Ganymede and Callisto) Facts about Ganymede Ganymede is the largest moon in the solar system, and is larger than the planet Mercury. If it were not orbiting as Jupiter’s second-largest moon, it could be considered a dwarf planet. Ganymede is the only moon in the solar system known to have a substantial magnetosphere. That implies there is something inside helping to generate a strong magnetic field. Like Europa, Ganymede is thought to have a subsurface ocean, overlying a liquid iron and nickel core. That core is what helps generate the magnetic field.

The surface of Ganymede is icy and covered with two main types of landscape: young, lighter regions and darker, older and cratered terrain. The dark areas appear to contain clays and organic materials. Ganymede was likely formed in place around the infant Jupiter in the early solar system. Several smaller worlds likely accreted together to make this moon. Ganymede has a thin atmosphere that appears to contain oxygen. This was confirmed by Hubble Space Telescope observations. The oxygen is likely freed as water ice on the surface is broken apart into hydrogen and oxygen by solar radiation. The first mission to explore Ganymede up close was Pioneer 10, followed by the Voyager missions, Galileo, and New Horizons. Several missions to explore Ganymede in more detail have been suggested, but most have been cancelled or are still on the drawing boards.

Sources: Touring Jupiter's Big Moons: Io, Ganymede, Europa, Callisto Touring Jupiter's Big Moons: Io, Ganymede, Europa, Callisto By SPACE.com Staff | November 22, 2010 10:41am ET Astrophysicists bring us up close with the large globes first glimpsed by Galileo as he observed Jupiter. Each is a very different world from its neighbors. credit : SPACE.com Europa — Facts And Information About Jupiter Moon Europa Europa: Facts About Jupiter's Icy Moon and Its Ocean By Elizabeth Howell, Space.com Contributor | June 13, 2016 10:20pm ET MORE Complex and beautiful patterns adorn the icy surface of Jupiter's moon Europa, as seen in this color image intended to approximate how the satellite might appear to the human eye. The data used to create this view were acquired by NASA's Galileo spacecraft in 1995 and 1998. Credit: NASA/JPL/Ted Stryk Europa is one of the Galilean moons of Jupiter, along with Io, Ganymede and Callisto.

Astronomer Galileo Galilei gets the credit for discovering these moons, among the largest in the solar system. Europa is the smallest of the four but it is one of the more intriguing satellites. The surface of Europa is frozen, covered with a layer of ice, but scientists think there is an ocean beneath the surface. The icy surface also makes the moon one of the most reflective in the solar system. Water plumes were spotted jetting from the moon in 2013, although those observations have not been repeated. Several spacecraft have done flybys of Europa (including Pioneers 10 and 11 and Voyagers 1 and 2 in the 1970s). The Galileo spacecraft did a long-term mission at Jupiter and its moons between 1995 and 2003. Both NASA and the European Space Agency plan missions to Europa and other moons in the 2030s. Facts about Europa Age: Europa is estimated to be about 4.5 billion years old, about the same age of Jupiter. Distance from the sun: On average, Europa's distance from the sun is about 485 million miles (or 780 million kilometers).

Distance from Jupiter: Europa is Jupiter's sixth satellite . Its orbital distance from Jupiter is 414,000 miles (670,900 km). It takes Europa three and a half Earth-days to orbit Jupiter. Europa is tidally locked, so the same side faces Jupiter at all times. Size: Europa is 1,900 miles (3,100 km) in diameter, making it smaller than Earth's moon , but larger than Pluto . It is the smallest of the Galilean moons. Temperature: Europa's surface temperature at the equator never rises above minus 260 degrees Fahrenheit (minus 160 degrees Celsius). At the poles of the moon, the temperature never rises above minus 370 F (minus 220 C). A translation of the key passages of Galileo Galilei's journal detailing his discovery of four moons orbiting Jupiter. Galileo made these sketches after discovering four moons orbiting Jupiter in January 1610. The moons, later named Io, Europa, Callisto and Ganymede, were the first discovered beyond Earth. Credit: NASA Discovery Galileo Galilei discovered Europa on Jan. 8, 1610.

It is possible that German astronomer Simon Marius (1573-1624) also discovered the moon at the same time. However, he did not publish his observations, so it is Galileo who is most often credited with the discovery. For this reason, Europa and Jupiter's other three largest moons are often called the Galilean moons. Galileo, however, called the moons the Medicean planets in honor of the Medici family . [ Photos: Europa, Jupiter's Mysterious Icy Moon ] It is possible Galileo actually observed Europa a day earlier, on Jan. 7, 1610. However, because he was using a low-powered telescope, he couldn't differentiate Europa from Io, another of Jupiter's moons . It wasn't until later that Galileo realized they were two separate bodies. The discovery not only had astronomical, but also religious implications. At the time, the Catholic Church supported the idea that everything orbited the Earth, an idea supported in ancient times by Aristotle and Ptolemy. Galileo's observations of Jupiter's moons — as well as noticing that Venus went through "phases" similar to our own moon — gave compelling evidence that not everything revolved around the Earth.

As telescopic observations improved, however, a new view of the universe emerged. The moons and the planets were not unchanging and perfect; for example, mountains seen on the moon showed that geological processes happened elsewhere. Also, all planets revolved around the sun. Over time, moons around other planets were discovered — and additional moons found around Jupiter. Marius, the other "discoverer," first proposed that the four moons be given their current names, from Greek mythology. But it wasn't until the 19th century that the moons were officially given the so-called Galilean names we know them by today. All of Jupiter's moons are named for the god's lovers (or victims, depending on your point of view). In Greek mythology, Europa was abducted by Zeus (the counterpart of the Roman god Jupiter), who had taken the form of a spotless white bull to seduce her. She decorated the “bull” with flowers and rode on its back to Crete. Once in Crete, Zeus then transformed back to his original form and seduced her. Europa was the queen of Crete and bore Zeus many children.

These images show the trailing hemisphere of Jupiter's moon Europa taken by the Galileo spacecraft at a distance of about 677,000 km. The left image shows Europa in approximately true color and the right image shows Europa in enhanced color to bring out details. The bright feature towards the lower right of the disk is the 45 km diameter crater Pwyll. Credit: NSSDC Photo Gallery Characteristics of Europa A prominent feature of Europa is its high degree of reflectivity. Europa's icy crust gives it an albedo — light reflectivity — of 0.64, one of the highest of all of the moons in the entire solar system. Scientists estimate that Europa’s surface is about 20 million to 180 million years old, which makes it fairly young. Pictures and data from the Galileo spacecraft suggest Europa is made of silicate rock, and has an iron core and rocky mantle, much like Earth does. Unlike the interior of Earth, however, the rocky interior of Europa is surrounded by a layer of water and/or ice that is between 50 and 105 miles (80 and 170 km) thick, according to NASA.

From fluctuations in Europa's magnetic field that suggests a conductor of some sort, scientists also think there is an ocean deep beneath the surface of the moon. This ocean could containsome form of life. This possibility of extraterrestrial life is one of the reasons interest in Europa remains high. In fact, recent studies have given new life to the theory that Europa can support life . The surface of Europa is covered with cracks. Many believe these cracks are the result of tidal forces on the ocean beneath the surface. It's possible that, when Europa's orbit takes it close to Jupiter, the tide of the sea beneath the ice rises higher than normal. If this is so, the constant raising and lowering of the sea caused many of the cracks observed on the surface of the moon. Obtaining samples of the ocean may not require drilling through the icy crust. In 2013, the Hubble Space Telescope identified geysers of water vapor spewing from the moon's south pole. The plumes subsequently disappeared, leading scientists to wonder if the features were a cyclical event. In 2014, scientists found that Europa may host a form of plate tectonics .

Previously, Earth was the only known body in the solar system with a dynamic crust, which is considered helpful in the evolution of life on the planet. Europa: Where life may evolve? The presence of water beneath the moon's frozen crust makes scientists rank it as one of the best spots in the solar system with the potential for life to evolve . The icy depths of the moons are thought to contain vents to the mantle much as oceans on Earth do. These vents could provide the necessary thermal environment to help life evolve. If life exists on the moon, it may have gotten a kick from deposits from comets. Early in the life of the solar system, the icy bodies may have delivered organic material to the moon. In 2016, a study suggested that Europa produces 10 times more oxygen than hydrogen , which is similar to Earth. This could make its probable ocean friendlier for life — and the moon may not need to rely on tidal heating to generate enough energy. Instead, chemical reactions would be enough to drive the cycle. Future exploration of Europa In 2013, the U.S. National Research Council's Planetary Science Decadal Review issued its 10-year recommendation for NASA's planetary exploration program.

Exploration of Europa was ranked as the highest-priority mission. Since then, NASA has been working toward a mission to Jupiter's icy moon . According to NASA, this mission — which would leave no earlier than the late 2020s — would perform several flybys of Europa with a spacecraft orbiting Jupiter. There will be nine scientific instruments on board including cameras, radar to peer beneath the ice and try to figure out its thickness, a magnetometer to measure the magnetic field (and by extension, how salty the ocean is), and a thermal instrument to search for signs of eruptions. The 45 flybys would range in height between 16 miles (25 km) and 1,700 miles (2,700 km). The European Space Agency also plans a mission to Europa and two other moons called JUICE, or JUpiter ICy moons Explorer. The mission is expected to launch in 2022 and arrive at Jupiter's neighborhood in 2030 for at least a three-year mission. Once it gets to Europa, the mission will look at organic molecules and other components that could make the moon friendly to life.

Also, the spacecraft will probe how thick the crust is, particularly over any active regions it finds. Additional reporting by contributor Nola Taylor Redd. Additional resources Moon and Jupiter close Sunday night! | Tonight | EarthSky Moon and Jupiter close Sunday night! By Bruce McClure in Tonight | April 17, 2016 Summer Triangle on winter evenings Tonight – April 17, 2016 – keep watching the moon and Jupiter! If you’ve been watching these past few days, you know the waxing gibbous moon has been moving closer to Jupiter in our night sky. The featured sky chart at top shows the moon’s position relative to Jupiter and the star Regulus as seen on the evening of April 17 from North America. In the world’s Eastern Hemisphere – Africa, Europe and Asia – the moon still shines close to Jupiter but is somewhat offset toward Regulus. Do you have a telescope? Wait until Earth’s moon moves away, then try to view Jupiter’s four major moons through your ‘scope. Or try tonight, because you can often see these moons in a moonlit sky. In their outward order from Jupiter, the moons are Io, Europa, Ganymede and Callisto.

But, as viewed from Earth on successive evenings, their order will be seen to change. As darkness falls over North America on April 17, 2016, Callisto and Europa will appear on one side of Jupiter, while Callisto will be on the other. Ganymede will be behind Jupiter, and not visible. Follow Jupiter’s moons with this chart from skyandtelescope.com. Enjoying EarthSky so far? Sign up for our free daily newsletter today! The inner three moons – Io, Europa and Ganymede – have a 4:2:1 orbital resonance. For every four times that Io orbits Jupiter, Europa orbits twice and Ganymede orbits once. Callisto is expected to join in several hundred million years from now, to create a 8:4:2:1 orbital resonance. Given that Io’s mean distance from Jupiter is 262,000 miles, we can figure out Europa’s distance by using Kepler’s third law of orbital motion, D3 = P2, where D = distance and P = orbital period. We know Europa’s orbital period (P) is twice that of Io.

So we can plug the number 2 into Kepler’s equation below to find out Europa’s distance relative to Io: D3 = P2 D x D x D = 2 x 2 D x D x D = 4 D = 1.5874 times Io’s distance from Jupiter Distance of Europa = 1.5874 x 262,000 = 415,898.8 miles Bottom line: Whether you enjoy the simple beauty of Kepler’s third law or the visual beauty of the heavens – or both – let the waxing gibbous moon be your guide to the planet Jupiter on April 17, 2016! BBC - Solar System - Ganymede (pictures, video, facts & news) Listen now 30 min Astronomer Paul Murdin asks if Jupiter's moon, Europa, might sustain biological life. Astronomer Paul Murdin explores if Jupiter's moon Europa might sustain biological life, an idea proposed by Arthur C Clarke in his novel 2010: A Second Space Odyssey.

About Ganymede Ganymede /ˈɡænᵻmiːd/ (Jupiter III) is the largest moon of Jupiter and in the Solar System, and the only moon known to have a magnetic field. It is the seventh satellite outward from Jupiter and third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. Ganymede has a diameter of 5,268 km (3,273 mi), 8 % larger than the planet Mercury, but its mass is only 45 % that of Mercury. Ganymede is 2 % larger than Saturn's Titan (second-largest moon of the Solar System). At 2.02 times the mass of the Moon, it is the most massive planetary satellite. It is the ninth-largest object in the Solar System, and the largest without a substantial atmosphere. Ganymede is composed of approximately equal amounts of silicate rock and water ice.

It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating. Ganymede's magnetic field is probably created by convection within its liquid iron core. The meager magnetic field is buried within Jupiter's much larger magnetic field and would show only as a local perturbation of the field lines. The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone).Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere associated with its atmosphere is unresolved. Ganymede's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610.

The satellite's name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus's lover. Beginning with Pioneer 10, spacecraft have been able to examine Ganymede closely. The Voyager probes refined measurements of its size, whereas the Galileo craft discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of all three icy Galilean moons, the probe is planned to enter orbit around Ganymede. Europa: Europa FAQ Europa FAQ Frequently Asked Questions about Europa Topics: Europa Missions Europa Why is Europa considered by many to be the most likely place in the Solar System to find life? Europa is the only other place in the Solar System, aside from Earth, for which there is very strong evidence suggesting a liquid water ocean is present today and is in contact with rock. This is important because life as we know it requires three key basic "ingredients": liquid water, an energy source, and organic compounds to use as the building blocks for biological processes. Europa could have all three of these ingredients.

Moreover, and its ocean may have existed for the whole age of the Solar System, giving life a chance the time to begin and evolve there. Atomic particles trapped in Jupiter's powerful magnetic field crash into Europa's icy surface with great energy, producing chemical compounds that could be used by living things. This process provides a possible energy source that mimics the life-supporting energy of Earth's oxygen atmosphere. Europa's seafloor could be heated by the moon's constant flexing, driving hydrothermal activity similar to undersea volcanoes in Earth's oceans, where some of the most primordial life forms thrive on energy derived from chemical reactions between water and hot rock. Aside from Earth and Europa, no other body in the solar system shows such strong evidence of all three basic "ingredients." Why is Europa such an attractive place to search for life, and can we tell from orbit or flybys whether there is life there? Europa's high-energy particle environment and the probability of a tidally heated subsurface ocean relatively close to the surface suggest that the moon could have all the conditions necessary to support life. From orbit or multiple flybys we can search for molecular signs of life that have been transported from Europa's ocean up to its surface.

Such signs of life are called biosignatures, and could include byproducts of metabolism. The science instruments included on a future mission to Europa would be designed to collect the chemical fingerprints of materials on the moon's icy surface, which researchers will examine for biosignatures. Scientists around the world are working together to determine what types of biosignatures to look for and how a spacecraft's instruments could best accomplish the task. However, ultimately a future landed spacecraft would be the most suited to directly search for biosignatures on Europa. Is there life on Europa? Given what we know about the ingredients necessary for life, Europa is certainly a very likely place in the Solar System to look for evidence of life. A future mission to Europa would likely seek such evidence by determining whether the moon harbors an ocean of liquid water below its icy surface. Such a mission would examine Europa's surface from orbit or during multiple flybys for the chance that we might be able to identify any chemical signs of life that might have been transported from the ocean. Why is there an ocean? Why isn't the whole moon frozen?

Tides on Europa, similar to the tidal interactions between Earth and its moon, cause Europa's ice shell and interior to flex during the course of its 85-hour orbit around Jupiter. On Io, such tidal flexing leads to intense heating, driving even more volcanic activity than occurs on Earth. The same source of heating can affect Europa's ice shell and possibly its interior. The heating is not enough to permit liquid water on the surface, but it should be sufficient to maintain a liquid water ocean beneath an outer ice shell. What is tidal heating? A moon that travels in an oval-shaped (or elliptical) orbit around its parent planet is repeatedly stretched and squeezed with every orbit as it moves closer to and farther from the parent planet, and this flexing produces heat. An analogy is the heat produced by repeatedly bending a paper clip back and forth. Moon orbits will tend to become less oval-shaped and more circular after a (geologically) short period of time, unless they are forced to be elliptical through gravitational interactions with other satellites. This is the case with Jupiter's moons Io, Europa, and Ganymede today. Tidal heating falls off markedly with distance from the parent planet.

This helps explain why Io is so volcanically very active today, but Europa is cooler, and Ganymede is least active today. "Warm" ice is much more easily squished than cold ice, so tidal heating concentrates in ice that is already warm; thus, there can be a feedback mechanism by which tidal heating produces warm ice, which in turn produces even more tidal heating. Why do we think that Europa has a subsurface ocean? There are several strong pieces of evidence that suggest an ocean exists on Europa: The magnetometer aboard the Galileo spacecraft detected signs of an induced magnetic field near Europa's surface, clear evidence for a conductive substance less than about 30 kilometers (about 20 miles) or so below. This is strong evidence for some amount of salty liquid below the surface. But the Galileo spacecraft was not designed specifically to test the hypothesis of a subsurface ocean on Europa. To know for sure whether Europa has an ocean, a spacecraft would need to carefully measure the tidal fluctuations of Europa's surface as it orbits Jupiter. This would require a spacecraft to either to go into orbit around the moon or make repeated flybys with the right geometries.

Europa's surface features (including bands, ridges, chaos terrain, and multi-ringed impact structures) suggest that there is warm, mobile, glacier-like ice at relatively shallow depths that sometimes has reached the surface. The presence of an ocean would make Europa's frozen surface flex more under its daily tides, cracking and warming the ice, to help explain its strange surface geology. For example, Europa's bizarre curved fractures (called cycloids) probably owe their origin to cracking in response to the flexing of Europa's icy shell on the very rapid time scale of its 3.55-day rotation. Creating large fractures in this way this requires large tides, best facilitated by liquid water. Also, Europa's multi-ringed impact structures suggest that Europa's largest impact scars punched all the way through the ice shell into an ocean. Judging by the pattern of the satellite's large-scale fractures, the surface of Europa may have "slipped" relative to its interior (processes called "nonsynchronous rotation" and "polar wander"). A subsurface ocean would greatly facilitate this slipping, allowing the ice shell to slide over the fluid ocean. Such slipping would be much more difficult if the ice shell was in direct contact with rock.

Is Europa's ocean like Earth's oceans? We don't really know, but we certainly think it is briny, with salts and minerals dissolved in it, like Earth's oceans. This hypothesis is based on theories for planet formation, which predict that Europa's water came from primordial material that is similar to some asteroids. The extraction of water from what is now Europa's rocky mantle would also have extracted minerals and salts from these rocks. Measurements by the Galileo orbiter support the likelihood of a salty ocean, because salt water is the most likely conductive material for generating Europa's induced magnetic field. How thick are Europa's outer icy shell and its possible ocean? Theory and observation indicate that Europa's icy shell is around 15 to 25 kilometers (10 to 15 miles) thick, overlying an ocean approximately 60-150 kilometers (40 to 100 miles) deep. Support for this hypothesis comes from observations of pits, domes, and spots on Europa's surface.

The size and spacing of the features suggests that they are due to churning within the ice shell, and theory suggests that such churning (called convection) can occur only if the shell is between greater than about 15 kilometers thick. Measurements of the height of domes on Europa (up to a kilometer or just over half a mile high) also suggest that the ice shell must be fairly thick for the domes to be so tall. Some scientists have argued that the ice shell might be thinner, only a few kilometers thick. Using radio signals to track the Galileo spacecraft as it passed by the icy moon provided insight into how material is arranged inside Europa. Those results suggested an outer layer of water and/or ice between 80 and 170 kilometers (about 50 to 100 miles) thick. The rest of the interior is probably made of up of rocky mantle and a metal core. Do "icebergs" really exist on Europa? Close-up pictures of Europa's "chaos" region, obtained by the Galileo spacecraft, show blocks or plates of the surface, several kilometers across, which have moved about relative to one another.

These have informally been referred to as "icebergs," but this term is a bit too suggestive, invoking visions of ice chunks floating in terrestrial polar seas. On Europa it would be extremely difficult (and perhaps impossible) to melt from a subsurface ocean all the way to the surface. Instead, the chaos areas may represent areas in which Europa's ice shell has partially melted above a large diapir -a blob of relatively warm ice - or an assemblage of diapirs. Understanding how chaos regions formed would be an important objective for a future mission to Europa. How cold is Europa's surface? Pretty darned cold. Temperature depends not only on distance from the Sun, but also on how reflective the surface is (a quality referred to as albedo), because bright surfaces are cooler and dark surfaces are warmer. On Europa, temperatures range from as high as about 140 Kelvin (about -210 degrees Fahrenheit) in dark material at the moon's equator to as low as about 50 Kelvin (-370 degrees Fahrenheit) in bright icy patches at the moon's poles. What causes the surface of Europa to look the way it does?

Europa's fractured appearance results from flexing of the moon's surface as it circles Jupiter on its oval-shaped orbit. As the moon's surface elongates toward Jupiter and then relaxes with each orbit, fractures open at the locations where the stress is highest. The fractures open, close, and slide past each other with each orbit. Some of warm and relatively mobile icy material and/or liquid water may be squeezed to the surface to form ridges. In the past, the surface appears to have pulled open along some cracks and ridges, allowing huge tracts of warmer icy material to well up into the new gap and creating banded patterns. Other features called "chaos" probably formed as slightly warmer blobs of ice and/or water migrated upward within the ice shell, eventually breaking apart the surface. What causes the differences in brightness and color on Europa's surface? Most of Europa's surface consists of bright water ice. The moon has reddish non-ice material along cracks and within areas where the surface has been deformed, such as the chaos regions. This reddish substance is associated with briny material. Although brines themselves are colorless, something mixed in is reddish in color, perhaps sulfur.

It appears that this material has come from the subsurface, either from the ocean or from pockets of briny material within the ice, and it might redden as a result of exposure to the radiation that affects its surface. Over time, Europa's surface brightens, possibly by radiation damage or by the deposition of frost. How old is Europa's surface, and how do we know? Galileo spacecraft imaging shows that Europa's surface is sparsely cratered, meaning that the surface is probably young. In other words, the longer a surface lies exposed to space, the more time it has to be hit by impacts, and the more craters it will display. Moon rocks collected by astronauts during the Apollo missions provide dates for major epochs of cratering and estimates of cratering rates in the Solar System. Simulations of comet and asteroid orbits indicate that it is primarily comets that slam into Europa and the other Galilean satellites. From the modeled and observed numbers of comets near Jupiter today, estimates can be made for the age of Europa's surface. By this method, planetary scientists estimate that Europa's surface has an average age of about 40-90 million years.

(The uncertainty in their measurement indicates that the age could be between 20 million years and 180 million years, with around 65 million years being the most likely age). This is extremely young by geological standards, which are often concerned with spans of time hundreds of millions to billions of years long, and suggests that parts of Europa could still be active today. Has Europa always looked the way it does? Probably not. The surface may have changed its appearance through time. Features called chaos seem to be the most recent, while those called bands appear to have formed further back in Europa's history. This suggests that the type of activity that deforms Europa's surface has changed over time. Io's vigorous volcanism provides circumstantial evidence that Europa is also active in the present era. It should be noted that Europa's visible surface is only about 65 million years old on average, which is extremely young in geological terms. Because Europa's surface is so young, we can only guess at what it looked like before the most recent period of time. Is Europa currently active (like Enceladus is)? The surface of Europa appears to be very fresh and young by geological standards, less than 100 million years old.

Because of tidal heating, Europa could have active venting of water vapor, similar to the active vents that spew vapor from the south pole of Enceladus. However, Europa is much more massive than Enceladus (about 400 times more) so its gravity would probably prevent vented material from reaching more than a few tens of kilometers above the surface. If three of Jupiter's moons - Europa, Ganymede and Callisto - are thought to have oceans, why is Europa most significant? Europa's ocean is relatively close to the surface, with active geological processes that might be able to transport into the ocean oxygen and other oxidizing chemical compounds (electron acceptors) generated by high-energy particles at the surface, and which could be used by living things. Water at the bottom of Europa's ocean may be in direct contact with warm rock, supplying hydrogen and other reducing chemicals (electron donors). The process, if it occurs, is similar to the energy cycle that supports life on Earth, with surface chemicals replacing direct energy input from the Sun. In contrast, Ganymede's and Callisto's oceans are around 150 km (100 miles) below the surface, and are sandwiched between ice layers.

The surfaces of those moons are probably not geologically active today. This means that any chemicals generated on their surfaces that might support life cannot be transported down into their oceans. Moreover, chemical nutrients cannot easily enter from the base of their oceans, since the oceans are not in direct contact with rock. Why does Ganymede have its own magnetic field? Why doesn't Europa? Like Earth, Ganymede probably has a molten iron core where convection-up-and-down circulation in a fluid portion of a metal core-generates the icy moon's magnetic field. The Galileo spacecraft did not detect an internally generated magnetic field at Europa, which indicates that if Europa has a metallic core, its core is probably not molten. The Galileo spacecraft did detect a weak magnetic field around Europa, nonetheless-one created within Europa as Jupiter's own powerful magnetic field sweeps past the moon. Why is the moon called Europa? Europa and the other three largest moons of Jupiter - Io, Ganymede and Callisto - are named for the lovers of Jupiter in Greek and Roman mythology. When Galileo Galilei discovered these four moons in 1610, he sought to name them the "Medicean Stars" after his wealthy patrons, the Medici family.

Several years later, astronomer Simon Marius wrote of the suggestion by his friend Johannes Kepler (a very famous astronomer himself) that the moons be named for Jupiter's mythical loves. The names we use today did not truly catch on for many years, however, and most astronomers referred to them in order of their distance from Jupiter: I, II, III and IV. It wasn't until around 1850, when Saturn's growing family of newly discovered moons complicated the numbering scheme for that planet, that astronomers finally started calling the moons of both giant planets by names of figures in the myths of Jupiter and Saturn. In the myth, Europa was a beautiful noblewoman who was abducted by Jupiter, the king of the gods. Jupiter appeared to her disguised as a white bull while she was gathering wildflowers by the sea shore. The bull appeared so calm and gentle that Europa placed flowers around its neck and climbed upon its back. At this point, the bull dove into the sea and carried her away to the island of Crete. When was Europa discovered? Galileo Galilei was the first person to publish an account of observing Jupiter's four largest moons, including Europa, in 1610.

Io, Europa, Ganymede and Callisto are often called the Galilean moons, in honor of the great Italian astronomer. The German astronomer Simon Marius claimed to have observed Jupiter's moons at about the same time as Galileo, and is commonly credit as being the co-discoverer of the moons. The discovery of these four moons circling Jupiter was an extremely important event in history. It provided evidence supporting the proposal by Polish astronomer Nicolaus Copernicus that all motion in the universe is not centered on our own world, and added support to the view that Earth and the other planets orbit the sun, a shift in thinking called the Copernican Revolution. What are "convection" and "diapirs"? Convection is the up and down circulation of material. With icy moons we are concerned with thermal convection, in which warmer (less dense) material rises and cooler (more dense) material sinks. This is the same kind of movement seen in a bubbling pot of thick soup or a liquid motion ("lava") lamp.

In the interiors of icy satellites, ice can be heated by the decay of radioactive material or by tidal activity, whereas the near-surface ice is generally very cold because the surface is far from the Sun. Even though ice is a solid material, it can actually flow slowly like a fluid. Provided that the consistency (or "viscosity") of the ice is not too stiff, warm ice tends to rise, and denser colder ice tends to sink. Warmer and thicker ice layers are more likely to undergo convection. Earth's mantle convects, with warm rock masses moving upward through colder (denser) rock over long periods of geologic time. A rising or sinking blob of material, such as a blob of warm ice, is called a "diapir" when it pierces and moves through another material, such as a layer of cold ice. A diapir can also form when a lighter material of one composition lies beneath a denser material of different composition, which is known as compositional convection. On Earth, salt commonly rises as diapirs through overlying denser sedimentary rocks, as beneath the Gulf of Mexico or in Iran. Radiation at Jupiter Why is there so much radiation at Jupiter?

Radiation at Jupiter is mostly from solar wind particles, atomic particles that stream out of the sun and past the planets. Jupiter's enormous magnetic field traps belts of subatomic particles from the solar wind. These move along magnetic field lines connecting to the poles of Jupiter. Some high-energy particles strip material from the surfaces of Jupiter's satellites, adding to the trapped particles. Volcanic plumes from Io spew sulfur and other atoms into the magnetic field as well, making the space near Jupiter a dangerous mix of different types of radiation. How do engineers design spacecraft to survive and operate in this environment? The estimated total radiation dose that a potential future mission's electronics would receive is far more than most ordinary electronics can bear. Therefore, a mission would use parts that are radiation tolerant, along with shielding that would reduce the radiation impacting the electronics. Aluminum, tantalum, and tungsten are the most common shielding materials, and these have been used successfully on other missions such as Galileo. What are the radiation belts at Jupiter like? Jupiter's radiation belts are donut-shaped regions that surround the giant planet, associated with its magnetic field, and are filled with trapped atomic particles (electrons and ions) moving with great speed and energy.

The tiny particles bounce back and forth within the doughnut-shaped region, creating a continuous "hail" that is collectively referred to as particle radiation. This kind of radiation can be very damaging to ordinary spacecraft electronics, and would do great harm to any living things (like astronauts) that went there unprotected. Jupiter's most intense radiation belt is a large region of electrons between 143,000 and 787,000 kilometers (89,000 and 489,000 miles) away from Jupiter. Earth has similar radiation belts, called the Van Allen Belts. The ability of the radiation belts to trap charged particles is related to the strength of the planet's magnetic field. Therefore, Jupiter's radiation belts are much more intense than Earth's because Jupiter's magnetic field is about 20 times stronger than Earth's. What causes the radiation belts? Ions and electrons are atomic particles that have an electric charge. These charged particles respond to forces from the planet's magnetic field, causing them to travel in an arc toward the planet's north or south magnetic pole. At a certain point along this path, the particles feel a force that causes them to reverse direction and start moving toward the other pole.

This happens again at a certain point on their journey in the opposite direction. In this way, the particles are forced to "bounce" back and forth, continually and at high speed, in an arc above the planet - essentially they are trapped in a sort of "magnetic bottle." The bouncing particles also slowly drift around the planet's middle, creating the donut-shaped region that we call a radiation belt. Europa Missions What robotic missions are planned for exploring Europa? NASA is studying mission designs that would tackle the most pressing questions about Europa: What are the characteristics of its ocean? How thick is the icy shell? Is there near-surface water within the ice shell? What is the global distribution of geological features? Is liquid water involved in surface feature formation? Is the icy shell warm and convecting? What does the red material on the surface tell us about ocean composition? How active is Europa today? What is the plasma and radiation environment at Europa? What is the specific nature of organics in salts at Europa? The Jupiter Icy Moons Explorer (JUICE) being planned by the European Space Agency (ESA) will address some of these questions, and conduct detailed investigations of Europa's sister moon, Ganymede.

Visit ESA's JUICE mission website > Could we send astronauts to Europa? The travel time and energy required are so large that it will be many decades before we have the technology and vehicles large enough to do the job safely. Even if we develop the ability to send people to Europa, the giant magnetic field of Jupiter traps belts of atomic particles that move at high speed and with great energy. Europa orbits Jupiter within these radiation belts, making it impractical (or perhaps impossible) for astronauts to safely work there without some very advanced form of shielding. Why does it take so long to get to the Jovian System and Europa? Jupiter is five times farther from the sun than Earth. It would be possible to get a spacecraft to the Jupiter system in about three years, but in such a case our current rockets would only be able to send a very small probe. Using flybys of the inner planets (Venus, Earth, and/or Mars) in a technique called a gravity assist allows us to send a much larger and more capable spacecraft to Jupiter and Europa.

After the last of its gravity assists by an inner planet, a Europa-bound spacecraft would still take about three years to reach Jupiter, just as a much smaller spacecraft would on a direct route from Earth. What are the planetary protection requirements for Europa? The principal planetary protection requirement for Europa is that inadvertent contamination of a Europa ocean by terrestrial organisms must be avoided, to a probability level of less than 1 in 10,000. Implementation of these requirements for missions that plan to fly by, orbit, or land on Europa is typically divided into what needs to be done on Earth before launch and what can be done after the spacecraft launches. The requirements on Earth are that the spacecraft is assembled and tested in a "cleanroom" - an extremely clean environment - and that it the number of microbes on the surface is reduced as much as possible prior to launch. All parts would have to be cleaned and/or sterilized before they are installed in the spacecraft, and the spacecraft assembly facilities would have to be monitored to understand the number and types of microbes that are present. Any problematic microbe species would have to be eradicated prior to launch. The Jupiter radiation environment would also help to eradicate microbes on the surface of the spacecraft.

Planetary Protection requirements for missions that plan to land on or impact Europa also require that an inventory of organic substances and the number and types of microbes found on the spacecraft is made prior to launch. How would NASA meet the planetary protection requirements for Europa? NASA calculates the probability of inadvertent contamination by conservatively estimating several factors: the number of microbes at launch; the probability of contaminating organisms surviving the lengthy trip to Europa; the probability of organisms surviving the high radiation field around Jupiter, which also affects Europa; how likely a spacecraft is to land on Europa (considered to be 100% for an orbiter that ultimately impacts the surface or for a lander); the mechanisms for transporting material from the Europa surface to its subsurface, where it might come in contact with water, and; organism survival and proliferation before, during, and after transport to the Europa subsurface. Will a lander mission eventually be sent to Europa, and what would it do? A lander mission is a future possibility. A mission to land a robotic probe on Europa's surface might focus on seismology and astrobiology.

An instrument called a seismometer would detect Europa quakes and determine whether there is liquid water at depth and how deep that water is (in the likely case that the satellite is seismically active). Certainly it would be important to sample the chemistry of the reddish non-ice material on Europa's surface to test for its compatibility with life and for the presence of any organic materials. Understanding this material probably requires digging or drilling about 10 centimeters (4 inches) below the surface, because the radiation at Europa chemically alters materials on the surface. The possibility of a lander melting all the way through Europa's ice shell to an ocean is probably a very long time in the future. Read more about the design of a potential Europa lander mission . Ganymede: Facts about Jupiter's Moon, Ganymede Interactive Solar System Model Ganymede Facts Ganymede is the seventh moon of Jupiter and the third of the Galilean moons. It is the largest moon in the solar system and is larger than the planet Mercury and dwarf planet Pluto . Ganymede is only slightly smaller than Mars and if it were orbiting the Sun rather than Jupiter , it would be classified as a planet itself.

Ganymede was discovered by Galileo Galilei on 7 January 1610 along with the three other Jovian moons Europa, Io and Callisto . Together they are referred to as the Galilean moons. The discovery of these four Galilean satellites are what eventually led to the understand that the planets in the solar system were orbiting the Sun and that Earth was not the centre of the solar system. Size of Ganymede compared to Jupiter Side by side comparison of the size of Jupiter vs it’s moon Ganymede Facts about Ganymede If Ganymede were orbiting the Sun it would be a planet. Ganymede is larger than both Mercury and Pluto, and is 2/3 the size of Mars with a diameter of 5,268 km (3,273 miles) – 8% larger than Mercury. It has the highest mass of all planetary satellites and has more than twice the mass of the Earth’s Moon. The moon is named after a Trojan prince in Greek mythology. Originally called Jupiter III by Galileo, Ganymede was named after a suggestion from German mathematician and astronomer Johannes Kepler.

In the Greek story Zeus, a counterpart of Jupiter in Roman mythology, carried the prince Ganymede to Olympus, where he became a cupbearer for the Olympian gods. It takes Ganymede 7.1 Earth days to orbit Jupiter at a speed of 39,165 km/h. The moon orbits at 39,165 km (24,321 miles) an hour and takes 7.15455296 Earth days to complete its orbit around Jupiter. Like most moons in the solar system, Ganymede is tidally locked to Jupiter and one side is always facing the planet. Interestingly, for every orbit of Ganymede, Europa orbits Jupiter twice and Io orbits four times. Ganymede is the only moon in the solar system known to have its own magnetic field. It is believed that the magnetosphere of Ganymede is likely to have been created through convection within the liquid iron core of the moon. The field is very small though and is barely noticeable because it is buried within Jupiter’s much larger magnetic field. The moon is composed of equal amounts silicate rock and water ice and has several layers. Ganymede is made of rocky material and water ice.

It has a metallic iron and iron sulfide core which is surrounded by a rocky mantle and a very thick icy crust. There may be an ocean layer of liquid water beneath the surface of Ganymede. An study published in 2014 suggests that there might be a stack of several saltwater ocean layers separated by different phases of ice as far as 124 miles below the surface of Ganymede. The lowest liquid layer may be right next to the rocky mantle below and could be sign for sustainable life on Ganymede. 40% of Ganymede’s surface is covered with highly cratered dark regions. These dark regions are believed to be from heavy impact by asteroids or comets and date back to around 4 billion years ago. The lighter regions of the moon’s surface are not quite so old and cover the rest of Ganymede. The cause of the light terrain’s disrupted geology is though to be the result of tectonic activity caused by tidal heating – a build up of friction, orbital and rotational energy that are dissipated as heat in the crust of the moons and planets involved. Ganymede has a thin atmosphere than contains oxygen.

The moon has a very thin atmosphere and it does contain small amounts of oxygen but there is not enough to support any form of life. Six spacecraft have visited Ganymede since 1973. Pioneer 10 and 11 were launched in March 1972 and April 1973 respectively and made successful flybys of Ganymede in December 1973 and December 1974. They sent back limited data with blurry images of the moon. Voyager 1 and 2 returned much more high quality and detailed images in 1979 which showed the grooved terrain and also discovered that Ganymede was larger than first thought – replacing Saturn’s moon Titan as the largest satellite in the solar system. The Galileo spacecraft in 1995 passed as low as 162 miles (261 km) over the surfaces of the Galilean moons, sending back a lot of new data such as the discovery of the magnetic field, the ocean beneath Ganymede’s surface, and the highest resolution close up images of the moon.

Finally, New Horizons in 2007 sent back images and made topography and composition maps of Ganymede before continuing on towards Pluto and the Kuiper Belt . Ganymede Profile

Question: Europa and Ganymede are moons of which planet?
Answer: Jupiter